Interaction of hydrophobic peptides with model membranes: slow binding to membranes and not subtle variations in pore structure is responsible for the gradual release of entrapped solutes.

Investigation of the mechanism of action of membrane-perturbatory peptides often involves monitoring the release of entrapped solutes from small unilamellar vesicles. Complete release of vesicular contents can take 15 min or more. Theoretical calculations suggest that the process should be of the order of seconds and not minutes. We have investigated the membrane-perturbatory abilities of hydrophobic peptides corresponding to regions of pardaxin that are important for toxin action. Peptide-induced release of entrapped carboxyfluorescein (CF) from lipid vesicles under various conditions was monitored by fluorescence spectroscopy. Several minutes were required for the complete release of CF. This has been shown to be due to lack of instantaneous and complete association by all the added peptide with the lipid vesicles. In addition, for a given peptide/lipid molar ratio, an increase in lipid concentration causes an increase in the rate of CF-release. It is likely that increased binding following a greater number of collisions between peptide and vesicles is responsible for this observation. A Fast Protein Liquid Chromatography assay confirms that a significant amount of peptide remains unbound from the vesicles. Other investigators have reported the requirement for a similar time span for the complete release of vesicular contents by pardaxin and several other membrane-perturbatory peptide toxins. The proposed reason for the delay in lysis may therefore be applicable to a large variety of membrane-perturbants. Thus, the assay of peptide-induced release of vesicular contents is likely to predominantly reflect only the rate of association of peptide with the membrane, and not more subtle variations in the nature of the pore formed.